Whisk together the flour, sugar, cocoa powder, and baking powder.
Add the milk, oil, and vanilla. Mix until smooth. Then stir in the nuts.
Transfer the batter to an ungreased 1-quart baking dish.
To make the pudding:
Whisk together the sugar and cocoa powder in a small bowl.
Gradually stir in the boiling water.
Pour over the cake batter. Do not stir.
Bake 30 minutes, or until a pick inserted into the center comes out clean.
Notes
Recipe slightly adapted from BH&G New Baking Book.
Cover leftovers in the baking dish or transfer them to an airtight container. They'll keep in the fridge for about 4 days. Reheat in the microwave or in a 300ºF (150°C) oven just until warmed through.